Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, classroom training, and for use in home theaters. In typical operation, multimedia projection systems receive analog video signals from an input device and convert the video signals to digital information to control one or more digitally driven light valves. Depending on the cost, brightness, and image quality goals of the particular projection systems, the light valves may be of various sizes and resolutions and may be employed in single or multiple display configurations.
One type of light valve found in projection systems is a digital micromirror device (DMD). A DMD uses individually controllable mirrors to selectively reflect light either through projection optics or towards a light-absorbing surface, based on image data. Because the DMD operates by selective reflection, the axis of illumination needs to be separated from the axis of projection.
This off-axis illumination could add to the expense and/or sacrifice the compactness of the system. Due to the reflective operation of the DMD, the illumination optics must present light to the same face of the DMD that the projection optics receive light from. This requires either complex optics at the DMD face, or it requires the elements of the two optical systems be placed at distances from the DMD sufficient to allow for unfettered presentation/reception of the light. The physical separation of the components of these prior art systems typically make up a significant portion of the total packaging volume of the system, while necessitating additional optical elements, such as relay lenses, to provide proper illumination at downstream components. Additionally, off-axis illumination could result in oblique illumination of the face of the light valve, which could present additional difficulties.
Other types of light valves, e.g., liquid crystal display (LCD) light valves, operate by selectively rotating polarized light. The light that is incident upon the LCD light valve must be polarized to one polarization state, which could result in increased complexity and decreased light transmission through the system.